Major auditoriums are generally equipped with built-in sound systems which are suitable for live performances of music as well as for other types of programs. In such a system there are a fixed number of loudspeakers at fixed locations. Depending upon the particular type of program, certain microphones may be relocated or turned off during some portions of the program. Sound levels are typically controlled through a volume control console that is operated by a sound engineer while the program is in progress. A sophisticated sound console will permit adjusting the input volumes that are accepted from the various microphones, and hence controlling the balance between them. In addition to controlling volume levels and balance, there may also be a provision for adjusting the amplification of the sound in various portions of the frequency spectrum. The control console may also provide for adjustment of the sound volumes that are being supplied to the various loudspeakers. It would be unusual, however, to adjust the respective directions in which the various loudspeakers are pointed, since the directional nature of the composite output pattern of the entire set of speakers is usually specifically designed to conform to the size, shape, and other acoustic characteristics of the particular auditorium.
For any performances which occur inside a building, the acoustical characteristics of the building such as echo and reverberation time must be considered. Any substantial amount of echo is undesirable because listeners hear the echoed sound out of phase with the original sound that is being received direct from the source. Reverberation time is also a factor--reverberation time being a measure of the time required for sound to fade away after the source of sound (loudspeakers) has been turned off. Both echo and reverberation time are diminished when an audience is present in the building, because clothing has the capacity to absorb sound rather quickly, and the clothing of a large number of people can absorb a great deal of sound.
When a live performance is presented in an out-of-doors environment, different considerations apply. Echo tends to be minimal. And reverberation time tends to be small, because there are no walls to restrain the sound. The sound therefore tends to flow endlessly outwardly like radio waves being transmitted into space. It then becomes of even greater importance that loudspeakers are properly placed and properly pointed so that the originally delivered sound reaches all the listeners with appropriate amplitude or volume, and free of phase differences which create conflicting sounds.